Recently another gene was discovered to lift the hearts of all those battling with their weight. Mice with a gene called mahogany can guzzle their way through a fat-laden diet that would otherwise be guaranteed to pile on the pounds - and not add an ounce. But the oddly-named mahogony is just the latest piece in a jigsaw that, when complete, should transform the way we think about weight, dieting and obesity.

Once upon a time the moral superiority of the skinnies was absolute. Being overweight was simply a matter of taking in more calories than you use up. Eat less and you'll weigh less was the slogan and it formed the basis of shelves-full of joyless diet books. But along with medical treatments and psychiatry, the genetic revolution is transforming our ideas about fatness.

Some weight researchers in the US now believe that genes are responsible for a huge 80% of our weight. At least a dozen genes have been identified and the number is rising rapidly. The latest, mahogany, was found by researchers at an American biotech company called Millennium Pharmaceuticals. It makes a protein that helps decide how much fat the body stores. Mice with a particular mutation in mahogany were able to maintain a healthy weight whether they ate a diet containing a super-guzzling 42% fat or a model-like 9% fat, when both diets contained the same amount of calories.

Researchers have already identified genes with names like ob, tubby, fat and agouti-yellow, all of which have mutations that cause weight-gain when eating a normal diet. You can begin to glimpse the complexity of what is going on here from the fact that adding a mutated mahogany gene will stop a mouse with a mutated agouti-yellow from packing away the pounds, but it doesn't affect mice with other mutations.

The genetic approach to obesity was born five years ago when Jeffrey Friedman at New York's Rockefeller University identified a protein called leptin that is made by the ob gene. Rats with the ob gene knocked out had no leptin and got very fat. For a while, this looked like a magic bullet. If some people got fat because they didn't have enough leptin, then leptin in the form of a pill could mean no more dieting. But things turned out not to be so simple. In fact, some very fat people have more leptin than the rest of us do.

What is now clear is that the control of our weight is the responsibility of a number of interlocking systems - one tells us how hungry we feel, another says when we are full, another decides how much fat gets stored, another how fast it gets burnt up and so on.

Each of them is controlled by a number of different genetic settings. So, for instance, there is a gene associated with red hair that also tells you when it's time to stop eating, but some mutations can make that 'stop' message much weaker, and you just keep on snacking.

Another set of recently discovered genes control what are known as 'uncoupling proteins'. These let you turn fat into heat instead of storing it. Animals with high levels of one of these proteins, called UCP3, don't put on weight so easily. Then last month there was that story about how people who fidget are likely to be slimmer, something that is also genetically controlled.

Findings like these suggest that there are actually many different sorts of obesity. Two people might both be a couple of stone overweight, but one might have problems with their messages of satiation, while the other one doesn't have enough UCP3 and so just keeps on laying fat down.

The rate at which you burn off calories sitting around doing nothing can vary enormously, according to Eric Ravussin, who works for the pharmaceutical giant Eli Lilly in Indianapolis. He's found that some people use up as little as 1,067 calories in a day, while others consume a huge 3,015.

Even the panacea for weight-loss - exercise - turns out to be much more variable in its effectiveness than is generally recognised. Some people do indeed benefit enormously - in terms of stronger hearts and muscles, more stamina and so on. But Claude Bouchard of Laval University in Sainte-Foy, Quebec, has found that other people show almost no improvement in weight or health despite exercising five times a week for 20 weeks.

So it looks as though our weight is as much a mark of individuality as our face, although that doesn't mean there is nothing we can do about it. It just means that in the future, when all these genetic connections have been mapped out, weight-loss regimes will be much more tailored to individual needs. One overweight person who responds well to exercise might be put straight on a treadmill, while another who is bad at burning off fat might be put on a low fat diet. Others might not bother to do anything at all.

'There are some people walking around, 60 to 80 pounds over-weight, who have normal blood-sugar and normal blood pressure,' says David West, obesity specialist at Parke Davis in Alameda, California. 'Their joints are fine, they don't have gallbladder disease and they don't seem to be a greater risk for cancer. We suspect that they have a particular set of genes that protect them from the normal effects of being fat.' However, until you can get a genetic printout of which bits of your own weight control network need targeting because they are too sluggish, the closest you'll come to a personalised weight-loss regime is to follow some combination of all the things we now know to be good for you - not just dieting.

Alternative flab fighting

Many people who try complementary medicine report weight-loss as a welcome side-effect of their treatment. Now a company called Iron Bridge have put together a video called Weight, which follows people visiting homeopaths, acupuncturists and other specialists, and looks at what happens to their weight.

Complementary therapists see excessive weight as a symptom that something else is wrong, rather than a problem to be tackled head on with calorie control, and they emphasise individual differences. A clinical nutritionist in the video, for instance, talks about the way that food allergies, wheat and dairy especially, can be behind weight-gain. Cut the offending foods out of your diet and pounds drop off without the need to eat any less, they claim.

Acupuncturists, on the other hand, concentrate on how effectively organs like the liver and the gut are doing their job. If they are sluggish for some reason -and it seems likely that genes will turn out to be a factor - then toxins can build up. When that happens the body stores more fat and water to dilute them. Again, tackling a more fundamental problem, in this case the well being of the liver, can often handle weight along the way without the need to diet.

About this article

Fat by nature

This article was published on
the Guardian website
at 11.23 EDT on Tuesday 11 May 1999.
It was last modified at 11.23 EST on Thursday 3 November 2005.
It was first published at 11.23 EST on Thursday 3 November 2005.